THE HEART 



45 



aspiratory effect of the air-tight thorax may be negatived by 

 the fact that the negative pressure may be still recorded with 

 the thorax open. In fact, the explanation of cardiac dilatation 

 has yet to be found. 



The Cardiac Impulse has been studied by means of an instru- 

 ment termed a cardiograph, which transmits the impulse of the 

 heart on the chest wall to a recording apparatus. Curves so 

 obtained are often difficult to interpret ; they are the graphic 

 record, not of one, but of a series of events, the chief of which are 

 variations in ventricular pressure and changes in volume. In 

 Fig. 20 is a cardiogram which shows a small elevation corre- 

 sponding to auricular contraction, followed by a large rise due 

 to ventricular systole, with a sudden and then prolonged drop 

 indicating relaxation of the ventricles. In the horse the impulse 

 of the heart occurs on the 

 cartilages of the fifth and 

 sixth ribs, close to the 

 articulation with the rib, 

 the centre of the shock 

 being the fifth intercostal 

 space (see Fig. 45, p. 105). 



The Capacity of the 

 Heart may be ascertained 

 by enclosing it in a chamber 

 termed a cardiometer , and 

 measuring the change of 

 volume during systole and 

 diastole. 



Observations so con- 

 ducted show that the 

 ventricle does not empty 



itself at each systole ; as much as one-third of the blood may 

 be left in it. Colin, many years ago, showed the same thing 

 for the horse, and stated that not more than two-thirds or three- 

 fourths of the ventricular charge was expelled. The quantity 

 of blood which the heart is capable of dealing with cannot be 

 ascertained by measuring the capacity of the chambers. Munk 

 gives the capacity of the horse's ventricle at 1 litre (176 pints), 

 equivalent, roughly, to 1 kilogramme (2*25 pounds) blood, and 

 states that each ventricle contains one-thirtieth of the blood in 

 the body, so that when both contract one-fifteenth of the total 

 blood is ejected. Both ventricles deliver the same amount of 

 blood, for there is as much entering the heart as there is 

 leaving it. 



Work of the Heart. — This may be calculated if we know the 

 amount of blood being discharged from the heart at each stroke, 



Fig. 20. — Cardiogram taken with Marey's 

 Cardiograph (Stewart). 



A, Auricular systole ; V. ventricular systole ; 

 D, diastole. The arrow shows the 

 direction in which the tracing is to be 

 read. 



